Active optics The Keck telescopes and the Hobby-Eberly telescopes require their mirror segments to be perfectly aligned. Sophisticated computer software can do this 10-20 times a second. Both the Gemini telescopes and the VLT telescopes have morphable mirrors (thin and floppy), which also need to be held perfectly in place. A hundred or more "actuators" constantly nudge the mirror (or mirror segments) back into perfect form. These adjustments are typically only about 1/10,000 the thickness of a human hair and are enough to keep starlight precisely focused and counter-act many effects of the atmosphere.

Imaging systems Gone are the days when professional astronomers use their eyes for observing! Early 1900s began with photographic plates. In the 1980s, charge-coupled device (CCDs) where invented (earlier by the military then declassified and picked up for use in astronomy). CCDs contain millions of separate detector cells, called pixels. Each pixel independently measures the light that falls on them. CCDs are highly sensitive, detecting about 90% of the light falling on them (as opposed to the eyes 1%) and highly linear in response. This allows an accurate measurement of flux, regardless of how bright or dim the source is. CCD technology is used in digital cameras.

Spectrographs Images are nice, but to understand astronomical sources, the light is typically broken up into its components for individual study. To the left is a typical design for a spectrograph. Important components include: pupil for isolating light from a single source, filter to block out unwanted light, collimator for focusing the light, grating for spreading out the light into its components, and a detector, usually a CCD, to image the spectrum.

A Spectrum The light is sent through a spectrograph, and hits the CCD spread out like a rainbow. However, a CCD is color-blind! Luckily, light of differing wavelengths will reflect at different angles off the grating. We determine the amount of light hitting the detector, as a function of wavelength, by seeing where on the detector the light hits and how much charge is registered on the CCD at that pixel location.